Traditional, non-shrink labels frequently are prepared from paper, foil, polypropylene film, and the like, and are applied to a container by way of a “roll-fed” process in which the label is simultaneously wrapped around and glued to the container. The adhesive is typically a hot melt adhesive such as, for example, a styrenic block copolymer or ethylene vinyl acetate, which is applied both to the label seam to adhere the label to itself (“seaming”) and to the container to hold the label in place.
Hot melt adhesives are thermoplastic materials which are solid at room temperature. When heated to a liquid or molten form, the hot melt adhesive can be applied to one or more surfaces that are joined together as the hot melt adhesive cools back to a solid. There are no special requirements for the adhesive for non-shrink labels, other than providing enough strength to hold the label in place during typical handling procedures. In addition, because non-shrink labels are dimensionally stable (i.e., do not shrink), the adhesive can be applied over a wide range of temperatures without risk of prematurely shrinking or curling the label seam.
The adhesives used for non-shrink labels generally are not satisfactory for shrink labels, however. Because shrink labels deform when heated above their “shrink onset” temperature (i.e., the temperature at which shrinkage occurs), the adhesive must be applied at a temperature that will avoid premature curling and shrinking of the label. Further, after a shrink label is seamed, it must pass through a heat tunnel in order to shrink the label onto the underlying container. In the shrink tunnel, the adhesive must be able to withstand elevated temperatures and the force from the shrink label, which will tend to pull the label seam apart. Hence, the adhesive seam for a shrink label typically will have greater strength than a seam in a non-shrink label, and should be able to maintain this strength at the elevated temperatures of a shrink tunnel.
The hot melt adhesives that are used now with roll-on shrink-on labels are limited to applications involving low shrinkage films (e.g., having 15% shrinkage or less), such as, for example, labels prepared from a polypropylene substrate. Because these films have low shrink force and shrinkage, separation of the seam and curling of the label do not occur.
By contrast, labels prepared from other polymers such as, for example, polyesters, polystyrene, poly(vinyl chloride), and poly(lactic acid), can have much greater shrinkage and shrink forces than materials that are now being used for roll-applied, shrink-on applications. For example, labels prepared from polyester polymers can have 20 to 80% shrinkage. This high shrinkage enables the shrink label to tightly fit against highly contoured containers and results in a more aesthetically pleasing package. New hot melt adhesives are needed for roll-applied shrink labels that provide sufficient bond strength to withstand high shrink forces and elevated temperatures and can be applied at a temperature that does not cause premature shrinkage or curling of the film.